People keep talking too much about jitter issues, but did less to make some kind of improvement. I decided trying to do something real – a working asynchronous I2S FIFO.

Before I go any further, I need to make something clear in advance:

1. FIFO is widely believed as one of the final methods to deal with jitter problems. But the asynchronous FIFO itself is just a technology which could provide a logic function to isolate the two clock domain. That means, for an I2S stream, digital audio data could pass through without any modification but the input clock was blocked and replaced with the new one. The new clock is totally independent from the old one except they should have the similar frequency. The FIFO logic itself does not influence the sound quality;

2. What really makes it sound better (or even worse) is the low jitter secondary clock. For example, if we have a new clock after the FIFO which comes with one digit ps jitter(RMS), we should get much better sound quality than before (the jitter level of the master clock from DIR chip usually around 50ps(RMS) level above corner frequency). So, we should pay more attention on the secondary clock and related interface. Everything connected to secondary clock has to be very carefully to deal with; otherwise new additive jitter will be introduced;

3. The size of the FIFO memory should big enough to meet overflow or empty time caused by the worst case of i2s input clock, but still has to be carefully selected to get the balance between the delay time caused by the working depth(usually half full) of the FIFO;

4. FIFO with low jitter secondary clock could only reduce the play back jitter for an audio DAC, but has nothing to do with the sampling jitter. I don’t have any idea if a CD was reordered with poor clock of the ADC.

The main logic functions could be achieved by a FPGA/CPLD chip together with a megabit SRAM. But the clock section has to use specialized clock/timing circuit, because the jitter performance of the output of FPGA/CPLD are far from what we want.

Hello!
When I was reading this I was really excited, that is exactly what I am "waiting for". Unfortunately I am far too bad, far too green to contribute anything, I wish I could. I really hope your FIFO project is making progress, really looking foward to following this thread, all the best!

Hi florianderton, thank you for your reply.
Me too. I'v been waiting for this kind of toy for many years. That is why finially decided to make it by myself. Using the I2S FIFO, you could upgrade your DAC, your CD player, Sound card and everything related the I2S or SPDIF stream with the better external clock, and you don't have to put up with the DIR recover clock any more. Hopefully I could bring you some good news step by step. I'd be happy to share the DIY experience with you guys. Welcome feedbacks. Ian

Sorry for the late update. I’d like this I2S FIFO has the following features:1. The FIFO section.Memory size: 4MB;.FIFO half-full delay time: 0.743 seconds (for 44.1 KHz Fs);.Half-full over flow time: 1486 seconds (at 44.1KHz input I2S stream with 500ppm frequency tolerance);.Smart depth control strategy is integrated.2. The I2S input section.3.3V LVTTL logic input level with 5V TTL tolerance;.Support 44.1KHz, 48KHz,88.2KHz, 96KHz,176.4KHz,192KHz Fs;.Accept 16bit, 24bit, 32bit definition.3. The I2S output section.3.3V LVTTL logic output level with 5V TTL tolerance;.Support 44.1KHz, 48KHz,88.2KHz, 96KHz,176.4KHz,192KHz Fs;.keep output definition as same as the input stream;.Master clock frequency: 128*Fs, 256*Fs (default) or 512*Fs.4. The low jitter clock generator section.Clock section is a standalone daughter board with the rubber ring hook up; .Master clock will output via coax cable and separated from I2S connector;.Clock board is upgradeable and could be easily replaced with the different type of clock/timing solutions to experience the different taste;.Multi-frequency clock board support Fs automatically switching function;.Might comes with I2S re-clock function;.Might support external clock only mode with clock board free;.Different type of clock board may develop and available for different applications or different users. 5. Extension.Could interface with the DIR/DIT board to become a SPDIF FIFO;.LVDS/ECL I2S output support is under consideration.6. Power supply.6V/500mA DC input power supply. Could also be easily powered by a standard 6V rechargeable battery for the better performance;.On board CLC filter;.Independent ultralow noise, high PSRR LDOs equipped for each sections of the circuit.

This project is not that much complicated, but still a lot of jobs and a lot of details. Some of them are done and some are still undergoing. The Verilog HDL coding seems a bit boring, but itís very fundamental to the project. Modules by module, state by state, signal by signal, have to go through everything one by one. CPLD/FPGA design going across two clock domains has to be tested with careful. Both functional and timing simulation is very important to make sure design target is achieved before running the real hardware.PCB layout is my favorite job. Not only because it makes you feel like very close to the actual board, but also itís a kind of artwork.I just got the prototype PCBs last weekend and Iím busy assembling them right now. Iím pretty sure I could show something on the next post.

I just finish assembling the I2S FIFO board. A lot of SMT components. By using some little tools, itís still OK for hand assembling, not that much difficult. So, finally I could show you some pictures of the board. This board has an I2S input, an I2S output and a clock input port. It could be driven by an external master clock or working together with a standalone clock board. This is a brand new clock and has no any business with input I2S signals. Iím gonna test it the next step. Just cross your finger.

Thanks Egberttheone. I'm a R&D engineer. My job focuses on medical electronics. But Iím an audiophile also. In the past couple decades of years, I did quite a few audio projects, include tube/solid amplifiers, CD transports, antique/full range speakers, DACs, PC HIFI and something else. Now, what Iím interested in the most is the digital audio. I always believe we should get the better sound if we could do something perfect. Talking about commercial operating, I donít have much idea right now. It quite depends. It depends on what the performance it could achieve and what kind of application it can do. The most important thing is how many people need it. So I think at current stage, I have to focus on the project and try to get more improvement. Have a nice weekend. Ian.